Compound and perpendicular diffusive transport of cosmic rays

A Chapman‐Kolmogorov equation description of compound transport of cosmic rays across the field, due to random walk of the magnetic field lines is developed. The probability distribution (pdf) for the particle transport across the field, P⊥, is given as a convolution of the pdf for the random walk of the magnetic field, PFRW, with the probability distribution Pp describing the particle transport relative to the random walking magnetic field. We show, that the Chuvilgin and Ptuskin model for compound diffusion, at sufficiently late times can be written as a fractional Fokker Planck equation, involving ordinary diffusion along the field and compound diffusion perpendicular to the field. The model is used to explain the dropouts in low energy solar cosmic rays observed by ACE spacecraft (Mazur et al.).